Chinese presence in the Indian Ocean has recorded a significant increase ahead of the Malabar naval exercise involving India, the United States and Japan. The Indian Navy has sighted more than a dozen Chinese warships, including submarines, destroyers and intelligence-gathering vessels, in the Indian Ocean during the last two months, government sources said.

The 10-day naval drills will begin in north Indian Ocean on July 10. More than 20 warships will take part in the exercise, including US Navy aircraft carrier USS Nimitz, Indian carrier INS Vikramaditya and Japanese Izumo-class helicopter carrier.

The naval drill will be bigger and more complex than all previous editions. China has been suspicious of the trilateral engagement and has even lodged protests over Japan’s participation in the past.

A Chinese intelligence gathering ship, Haiwingxing, is understood to have sailed into the ocean in June-end. Strategic experts linked the Haiwingxing’s deployment to the forthcoming naval exercise. The navy’s satellites, surveillance planes and surface warships have also sighted Luyang III class destroyers, hydrographic research vessels and tankers. The presence of a submarine in the region has been confirmed by the presence of Chongmingdao, a Chinese navy submarine support vessel, the sources said.

Anti-piracy patrols and freedom of navigation are the reasons cited by China for its increased presence in the Indian Ocean, forcing New Delhi to tighten surveillance of the strategic waters, government officials said.

“Chinese activity in the Indian Ocean has touched a new high in recent months. We are using our surveillance assets quite extensively to monitor their movement,” a navy source told HT on Wednesday.

The sightings assume significance as the two militaries are in a three-week-long standoff at an India-China-Bhutan tri-junction close to the Sikkim border.

Beijing has raised the pitch and its media are warning of a war, demanding India withdraw its troops from the Doklam, a disputed region.

The Chinese deployment in the pirate-infested Gulf of Aden, one of the world’s most important shipping routes, accounts for four to five warships at any given time.

The periodic rotation of these units gave the Chinese navy the opportunity to deploy a variety of assets in the region, officials said. “Chinese submarines have been spotted in the region. Such assets are not used for anti-piracy operations. They are clearly making their presence felt in the region,” a naval officer said on condition of anonymity as he is not authorised to speak to media.

The Indian Navy has been present in the Gulf of Aden since October 2008 and so far, 64 warships have been involved in anti-piracy operations.

The officials said China has been deploying submarines in the Gulf of Aden regularly since 2014. “The pattern is they deploy a submarine for three months followed by a three-month break,” the officials said.

The imminent commissioning of a Chinese base and support facility at Djibouti in Horn of Africa will boost Beijing’s ability to sustain naval units in the Indian Ocean.

Sources said Chinese navy research vessels have been mapping the region for both military and economic purposes.

China has stepped up activities in the Indian Ocean where it is building ports and other infrastructure in Sri Lanka, Bangladesh and Pakistan. Indian Ocean figures prominently in President Xi Jinping’s ambitious One Belt, One Road initiative to build a new Silk Route.

New Delhi is buying 22 Guardian unarmed drones from the US to keep a close watch on the Indian Ocean. The purchase cleared by the US during Prime Minister Narendra Modi’s recent visit to the US has caused unease in China.